Olefin polymerization catalysts comprising a metallocene and an aluminum alkyl component were first proposed in about 1956. Australian patent 2436/88 proposed for use as a polymerization catalyst a bis(cyclopentadienyl) titanium, zirconium, or vanadium salt reacted with various aluminum alkyl compounds. Although some of these catalyst complexes were capable of polymerizing ethylene, the level of catalytic activity was insufficient for commercial uses.
Later it was found that certain metallocene catalyst systems such as bis-(cyclopentadienyl) titanium or zirconium dialkyls in combination with aluminum alkyl and water cocatalyst formed catalyst systems for the polymerization of ethylene. Such catalyst systems have been discussed in German Patents DE 2,608,863 and DE 2,608,933 both published on Sep. 9, 1977 and European Patent Application No. 0035242 published Sep. 9, 1991, however, these catalyst systems are commercially useful for the polymerization of ethylene and can control the molecular weight of a polymer product, other than by the addition of hydrogen, by controlling the reaction temperature or the amount of cocatalyst.
Further, in order to realize the benefits of such catalyst systems, one must use or produce the required cocatalyst component, alumoxane, which is produced by reacting an aluminum alkyl with water. The reaction of an aluminum alkyl with water is very rapid and highly exothermic because of the pyroforic nature of this reaction.
Alumoxane as a cocatalyst component has been separately prepared by one of two general methods. Alumoxanes may be prepared by adding extremely finely divided water, such as in the form of a humid solvent, to a solution of aluminum alkyl in benzene, other aromatics, or aliphatic hydrocarbons. The production of alumoxane by such procedures requires the use of explosion-proof equipment and extreme care in monitoring reaction conditions in order to reduce potential fire and explosion hazards. For this reason, it has been preferred to produce alumoxane by reacting aluminum alkyl with a hydrated salt, such as hydrated copper sulfate.
Several commonly-owned U.S. patents have addressed the above problems in the prior art, for example, U.S. Pat. Nos. 4,912,075, 4,937,301, 4,925,821, 4,937,217, 4,935,397 and 5,006,500, all of which are hereby fully incorporated by reference.
U.S. Pat. No. 5,006,500 describes procedures by which an alumoxane is safely produced directly on a silica support by reacting an alkyl aluminum with water-containing silica. Such alumoxane-silica product is then reacted with a metallocene to yield a supported metallocene-alumoxane catalyst useful in the polymerization of olefins. However, homogeneous catalyst components comprising titanium and zirconium metallocenes produce polymers, polyethylenes, having a typically narrow molecular weight distribution (MWD), i.e. M.sub.w /M.sub.n.
The above cited references neither disclose polyethylenes having a broad molecular weight distribution nor how to obtain such polyethylenes. Although there are many commercial applications for polyolefins with a narrow molecular weight distribution as a desirable property, for example to ease the process ability of polyolefins into finished a form, it is often desirable to have a polyolefin with a broader molecular weight distribution.
Several methods in the past have been directed toward making broader molecular weight distribution polyolefins. U.S. Pat. No. 4,310,648 discloses a catalytic reaction product of a titanium compound, a zirconium compound, an organomagnesium compound and a halide source. The reaction product (a heterogeneous catalyst), when employed in combination with aluminum alkyl, is useful for the production, at high activity, of broad molecular weight polyethylenes.
It is sometimes also desirable to have a polyethylene having a narrow molecular weight distribution and a high molecular weight. For example, U.S. Pat. No. 4,361,685 discloses the use of organic soluble chromium and zirconium compounds employed in combination with a supported catalyst system comprising an organometallic activating agent and a trivalent or tetravalent titanium compound. The polymers obtained have a high molecular weight and a narrow molecular weight distribution.
In "Molecular Weight Distribution And Stereoregularity Of Polypropylenes Obtained With Ti(OC.sub.4 H.sub.9).sub.4 Al.sub.2 (C.sub.2 H.sub.3).sub.3 Catalyst System"; Polymer, Pg. 469-471, 1981, Vol. 22, April, Doi, et al. disclose propylene polymerization with a catalyst, which at about 41.degree. C. obtains a soluble catalyst and an insoluble catalyst fraction, one with a "homogeneous catalytic centre" and the other with a "heterogeneous catalytic centre". The polymerization at that temperature obtains polypropylene having a bimodal molecular weight distribution. Also, U.S. Pat. No. 4,931,417 describes the use of bis(methyl-t-butyl cyclopentadienyl) zirconium dichloride to obtain atactic polypropylene.
U.S. Pat. No. 4,808,561 discloses a metallocene polymerization catalyst having cyclopentadienyl rings substituted with hydrocarbyl groups such as alkyl, aryl, alkylaryl, or arylalkyl groups. However, this patent does not teach a method of making broader molecular weight distribution polyolefins.
U.S. Pat. No. 4,935,474 discloses a method of obtaining polyethylene having a broad molecular weight distribution utilizing a catalyst comprising (a) at least two different metallocenes each having different propagation and termination rate constants for ethylene polymerizations and (b) an alumoxane. However, this U.S. Patent does not disclose or suggest a method of controlling the value of molecular weight distribution.
In spite of the methods known in the past for polymerizing broader molecular weight distribution polyolefins, there still exists a need in the art for an improved catalyst system and methods for making high quality polyolefins which also provide control over the broadened molecular weight distribution.